The ability to control the interaction of polyelectrolytes, such as DNA or proteins, with charged surfaces is of pivotal importance for a multitude of biotechnological applications. Previously, we measured the desorption forces of single polymers on charged surfaces using an atomic force microscope. Here, we show that the adhesion of DNA on gold electrodes modified with self-assembled monolayers can be biased by the composition of the monolayer and externally controlled by means of the electrode potential. Positive potentials induced DNA adsorption onto OH-terminated electrodes with adhesion forces up to 25 pN (at +0.5 V versus Ag/AgCl), whereas negative potentials suppressed DNA adsorption. The measured contributions of the DNA backbone phosphate charges and the doubly charged terminal phosphate on adsorption agreed with a model based on the Gouy-Chapman theory. Experiments on an NH(2)-terminated electrode revealed a similar force modulation range of the coulomb component of the desorption force. These findings are important for the development of new DNA-based biochips or supramolecular structures.
The development of single-molecule techniques has afforded many new methods for the observation and assembly of supramolecular structures and biomolecular networks. We previously reported a method, known as the single-molecule cut-and-paste approach, to pick up and deposit individual DNA strands on a surface. This, however, required pre-functionalization of the surface with DNA strands complementary to those that were to be picked up and then deposited. Here we show that single molecules of double-stranded DNA, bound to the tip of an atomic force microscope, can be deposited on a bare gold electrode using an electrical trigger (surface potential cycling). The interactions between the DNA and the electrode were investigated and we found that double-stranded DNA chemisorbs to the gold electrode exclusively at its end through primary amine groups. We corroborated this finding in experiments in which only a single adenosine nucleotide on a polyethylene glycol spacer was 'electrosorbed' to the gold electrode.
Polymer-surface interactions provide a basis for nanoscale design and for understanding the fundamental chemistry and physics at these length scales. Controlling these interactions will provide the foundation for further manipulation, control, and measurement of single molecule processes. It is this direction of control over nanoscale polymer-surface interactions that we explore with electric glue. The adhesion between surfaces and single molecules is manipulated based on an externally controlled potential in electric glue.
In atomic force microscopy (AFM) a sharp cantilever tip is used to scan surfaces at the atomic level. One further application is force spectroscopy, in which force-distance curves between binding partners located on the cantilever and substrate surface are determined. This requires specifically immobilized molecules. Herein we describe the covalent binding of single adenosine and thymidine nucleotides on an amino-PEGylated cantilever tip by the phosphoramidite method. Force-distance curves between these cantilever tips and gold surfaces were recorded. The rupture forces of the coordination bond between the primary amine of adenosine and the undercoordinated gold atoms were determined to be 145 pN, which is in agreement with previously published data. The force-distance curves of thymidine-functionalized tips did not show rupture events, because this nucleotide does not possess a primary amine function. Nucleotide-functionalized tips could aid in the understanding of binding mechanisms of nucleotide binding molecules such as polymerases immobilized on surfaces or membranes.
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